1. Field of the Invention
The present invention relates to a process for producing plated steel sheets such as those used for building materials, air conditioners and hot water equipment, and automotive steel sheets, which require high strength, good drawing workability, and high corrosion resistance.
2. Description of the Related Art
Plated steel sheet is usually produced by the following steps. A slab is rolled into a steel sheet by hot rolling, and a layer of iron oxide (referred to as a scale hereinafter) generated on the surface of the steel sheet during the hot rolling is removed by pickling equipment. Then, after being subjected to cold rolling and recrystallization annealing depending on the quality required for the steel sheet under production, the steel sheet is coated with a plating layer by a continuous hot dipping apparatus or an electroplating apparatus, for example, thereby producing a plated steel sheet. In the above process, if the scale generated on the surface of the steel sheet during the hot rolling is not removed, the scale would impede the plating process by promoting peeling-off of the plating layer and decreasing plating adhesion (i.e., adhesion of the plating layer to the steel surface). Also, the process including the step of recrystallization annealing after cold rolling is effective in producing a steel sheet superior in workability such as elongation and drawing characteristics.
To improve the above-stated conventional process for producing plated steel sheet, various approaches have been attempted so far. For example, Japanese Unexamined Patent Publication No. 6-145937 and No. 6-279967 disclose a technique which omits the steps of pickling and cold rolling, primarily to lower the cost. Specifically, those Publications propose that a hot-rolled steel sheet be subjected to a reducing process in a reducing gas atmosphere gas without removing the scale on the surface of the hot-rolled steel sheet, following which the steel sheet is plated by hot zinc dipping. Also, Japanese Unexamined Patent Publication No. 9-143662 and No. 9-217160 disclose a method for improving adhesion of a plating layer to the scale by causing cracks in the scale on the surface of a steel sheet with a tension leveler or the like prior to the reducing process. However, none of the above Publications mention the deterioration of workability which may result from omission of the cold rolling step. Further, Japanese Unexamined Patent Publication No. 6-145937 includes no description about adhesion of the plating layer. Japanese Unexamined Patent Publication No. 6-279967 improves adhesion of the plating layer by using a hot-rolled steel sheet on which a thin scale is deposited to a thickness of 1.1-4.6 .mu.m, but does not disclose a practical method for obtaining the thin scale. With the method disclosed in Japanese Unexamined Patent Publication No. 9-143662 and No. 9-217160, because cracks are generated in the scale prior to the reducing process, the adhesion force between the steel sheet and the scale is lowered, resulting in a danger that the scale may peel off during the reducing process and drop in the furnace or deposit on feed rollers, thus giving rise to flaws on the steel plate.
On the other hand, if steel of the type that contains an easily oxidized component such as Si and Mn is employed to increase the strength of plated steel sheet in the conventional production process, there arises a problem in that such an easily oxidized component becomes oxidized during annealing before the plating step, and is so concentrated on the surface of the steel sheet that the reaction between the steel sheet and the molten metal is impeded during the plating process and a bare spot results.